To form a thin film of a metal oxide on a substrate utilizing a sol-gel method, a metallic alkoxide is used as a raw material, and a precursor sol of the metal oxide is prepared by hydrolyzing and polymerizing the metallicalkoxide. The sol thus obtained is then applied to the surface of the substrate, and after forming a gel of the metal oxide on the surface of the substrate, a film of the gel is subject to a heat treatment at an appropriate temperature. At the time of forming an In.sub.2 O.sub.3 --SnO.sub.2 thin film on the surface of the substrate utilizing a sol-gel method, indium alkoxide and tin alkoxide are used as starter material.
Generally, as the hydrolysis of the metallic alkoxide such as indium alkoxide, tin alkoxide, etc. takes place very quickly, it is difficult to prepare a homogeneous sol capable of achieving a homogeneous film formation. To cope with this, there is a method in which concentration of metallic alkoxide is extremely reduced to restrain the speed of hydrolysis of the metallic alkoxide. By employing such a method, it is certainly possible to achieve a film formation of homogeneous sol. However, in such a method, thickness of the thin film obtained by one film formation process becomes extremely small, and this method is not suited for practical use.
As mentioned above, when concentration of the metallic alkoxide is high, it is difficult to prepare any homogeneous sol from which a homogeneous film can be formed. On the other hand, when concentration of the metallic alkoxide is reduced to make it possible to form a homogeneous film, thickness of the thin film obtained by one film formation process is excessively small, which is not suited for practical use. Moreover, generally, the sol obtained by hydrolyzing the metallic alkoxide is unstable such that change in viscosity may occur, precipitate may be produced, or gelation takes place, due to storage for a long time. These problems occur in the same manner at the time of forming the In.sub.2 O.sub.3 --SnO.sub.2 thin film utilizing a sol-gel method in which indium alkoxide and tin alkoxide are used as starter material.
As a further method for obtaining a sol from which a film is formed while increasing the concentration of metallic alkoxide and restraining the speed of hydrolysis, several attempts for stabilizing the metallic alkoxide by adding some organic compound capable of being multidentate have been heretofore proposed. For example, it was reported that in the formation of an alumina thin film using aluminum-s-butoxide as a starter material, .beta.-diketone is effective (Article 97,396(1989) by Japan Ceramics Association), and that in the formation of a titania thin film using titanisopropoxide as a starter material, 1,3-butanediol is effective (Article by Dr. Hisao Koshiba of Toyohashi Technological and Scientific College, March, 1993), diethanol amine is effective (Article 23, 2259 (1988) by Journal of Materials Science), and .beta.-diketone is effective (Article 97,213(1989) by Japan Ceramics Association) It is also reported that in the formation of a zirconia thin film using zirconium-n-butoxide as a starter material, employment of diethylene glycol is effective (Article 95,942(1987) by Ceramic Industry Association). Further, it was reported in the Articles 74,1407(1991) by Journal of American Ceramics Society and 25, 3960 (1990) by Journal of Materials Science that employment of .beta.-diketone and alkanol amine is effective in the composition of a composite oxide such as PbTiO.sub.3 and Pb(Zr, Ti)O.sub.3.
In the Physics of Tin Film, 5, p87(1969) and in the Academic Press, a method for producing an oxide film utilizing hydrolysis of various inorganic salts such as chloride, sulfate, nitride, ammonium salt and aqua-complex was reported. Further, it was disclosed in the Article 102,200(1994) by Japan Ceramics Association that, to prepare In.sub.2 O.sub.3 --SnO.sub.2 sol being a composite oxide, indium nitrate and tin chloride are used instead of metallic alkoxide.
However, in the method for restraining the speed of hydrolysis of the metallic alkoxide by stabilizing the metallic alkoxide such as indium alkoxide, tin alkoxide, etc. by adding some multidentate compound, it is certainly possible to prepare easily a homogeneous sol suited for film formation, but a lot of organic substances of high boiling point difficult to be decomposed at a high temperature exist in the sol or gel film. As a result, to remove such organic substances, it is necessary for the gel film to be heat-treated at a high temperature of about 500.degree. C. Further, since a lot of organic substances exist in the gel film, when heat-treating the gel film, reduction in weight of the film is large. In other words, a large number of pores are produced as the result of removing the organic substances from the gel film, which eventually results in any defect of the obtained thin film of metal oxide such as In.sub.2 O.sub.3 --SnO.sub.2. Moreover, to remove the pores in the film, an additional energy is required for elaboration of the thin film and, therefore, to obtain an In.sub.2 O.sub.3 --SnO.sub.2 thin film having desired characteristics, a burning at a temperature of not lower than 600.degree. C. is usually required.
Furthermore, any of the mentioned methods utilizing a metallic salt is essentially a thermal decomposition method, and therefore a lot of problems may occur in the aspect of film quality after the heat treatment.